1. Field of the Invention
The present invention relates to an improved free radical (co)polymerization of ethylene, and, more especially, to the preparation of copolymers of ethylene and at least one ester of acrylic or methacrylic acid; the final polymers are particularly well adopted for the coating of metal substrates.
2. Description of the Prior Art
Among the ethylene polymers and copolymers prepared by a free radical route, those having a fluidity index ranging from about 3 to 10 dg/min are well known to be particularly suitable for the deposition of thin film coatings having a thickness ranging from 10 to 500 .mu.m. For such application, two properties are essential to provide a suitable polymer or copolymers. These are the industrial drawing rate thereof, on the one hand, and the "neck-in" on the other. By "neck-in" is intended the contraction in width thereof, defined by the ratio. EQU [(width of the die)-(width of the film)]/(width of the die).
The known processes for the free radical polymerization of ethylene are typically processes which are carried out under high pressure. In this type of process a chain-transfer agent is sometimes employed, generally hydrogen, butane, propane or a lightweight .alpha.-olefin, such as propene or but-1-ene. In the copolymerization of ethylene with at least one ester of acrylic or methacrylic acid, an alkane, an olefin, an aldehyde or a ketone may also be used as the chain-transfer agent.
In the known processes, such as, in particular, those described in EP-A 174,244 and EP-A 177,378, however, it is not possible to attain drawability and "neck-in" levels which universally permit the use of the final copolymers for all coating applications.